Photolithography or microlithography apparatus are widely used in the fabrication of microelectronic semiconductor devices and other microdevices. In photolithography, an optical system directs light energy to record a pattern at high resolution and with precise registration onto a photosensitive layer formed on a silicon wafer or other substrate. Continuing improvements in miniaturization place increasingly more challenging demands on the performance and accuracy of the optical system used for this function.
Microlithography optical systems are fairly large and complex, containing a number of optical elements. In a typical “stepper lens” arrangement used for microlithography, stacked annuli lens assembly is used, as described, for example, in U.S. Pat. No. 5,428,482 entitled “Decoupled Mount for Optical Element and Stacked Annuli Assembly” to Bruning et al. Each lens element is accurately mounted within a cylindrically shaped cell, typically of stainless steel. Each cell is fabricated to extremely tight tolerances, with faces ground flat and parallel. When the lens is assembled, each successive cell is bolted to the face of its adjacent cell with no adjustment possible other than a small centering motion in X and Y. Once all the cells have been assembled, the entire lens is tested and any unwanted aberrations or image defects are discovered. Commonly, a lens can be completely assembled before it is determined that one or more of the elements may need to be moved slightly in the Z or axial direction in order to correct a measured optical defect. To accomplish this, the lens must be disassembled and new spacers inserted, whereupon the lens is reassembled, carefully making all the centering adjustments again.
Achieving correct magnification and focus are critical for obtaining precise layer-to-layer registration and submicron resolution with photolithographic optics used for device fabrication. Focus adjustment is usually enabled by displacement of an optical element along the optical axis, conventionally the z-axis, with no translation in the orthogonal x or y axes. For example, in order to properly adjust magnification or focus, it is often necessary to move specific components of the optical system to specific positions along the optical axis. In obtaining this movement, it is important to minimize or eliminate inadvertent movement of other components of the optical system.
Where lens axial adjustment may be necessary in a stacked annuli arrangement, solutions that take advantage of balanced or kinematic constraining forces, using springs and flexures for example, can be more promising for high precision adjustment applications than are static solutions. However, proposed solutions of this type for providing pure axial translation adjustment are typically highly complex, often requiring precision fabrication and assembly of multiple interconnecting parts. As just one example, in the embodiment described in U.S. Pat. No. 6,538,829 entitled “Optical Element Mount Comprising an Optical Element Holding Frame” to Rau et al., an optical mount for adjusting two components relative to each other is shown. The mechanism described in the '829 Rau et al. disclosure employs a fairly complex network of flexures and hinges for providing this type of axial translation adjustment.
With any type of solution for axial adjustment, even the slightest parasitic effects or asymmetries of construction can compromise the purity of motion demanded for lens adjustment in high-resolution photolithography. Materials used for the different components and their fasteners must be carefully specified to minimize thermal effects due to differences in coefficients of thermal expansion (CTE).
Overall, conventional lens mounting methods are likely to cause overconstraint and other problems affecting purity of motion that limit their usefulness for photolithography applications. While various solutions for axial positioning of optical elements have been proposed, there remains a need for an optical assembly mount that allows adjustment of position for individual optical components along the optical axis, but inhibits rotation and movement along axes other than the optical axis, uses a relatively small number of parts, and provides the level of performance necessary for use with optical assemblies for microlithography and other precision optical and positioning applications. Further, it would be advantageous to provide a solution that is capable of monolithic fabrication.